Vibration/Shutter at low RPM's on a Suzuki 225 w/ Cracked Lower Unit

[Zac Penn]

I have read the reasoning behind the crack (corrosion building up between the propshaft carrier nut and threads) and I am going to get mine fixed but I had a few questions 1st.

My crack is quite substantial but it hasn't made it up to the gear case yet. The fluid is perfectly blue with no water intrusion.

Since the crack is large the propshaft carrier nut has to be out of alignment from where it was at the factory. Do you think this is causing my vibration/shutter at low RPM's? If you go above 3000 rpm then it smooths out and you feel nothing. If the motor is in neutral then you feel nothing.

My plan is to drill a relief hole right at the end of the crack, then soak down the carrier nut with PB Blaster for a day or so. Then back out the carrier nut so hopefully I can get the crack to mesh back together as close as possible. Then have my metal shop weld it back together and grease up the carrier nut and threads so it doesn't happen again.

If you don't think this would be my shutter issue, then maybe I will just drill the relief hole in the crack and leave it alone and search for the shutter somewhere else.

 

[Zac Penn]

Okay so here is an update...
I found another lower unit that does not have the crack and is in good condition. I drained the old fluid, refilled and replaced the water pump, then installed onto my motor. I took it for a test drive and the same thing is still happening.

Shift into forward and idle and you are fine. Increase the revs to about 1500 (my control box only either idles, or jumps straight to 1500) and you can feel a shudder throughout the whole boat. The shudder remains until about 2300 rpm's and then the shudder goes away. The only problem with this is I am going too fast and create too much wake for "No Wake Zones".

So if the shudder is not being caused by the lower unit, then do we have any ideas?

 

[99yam40]

cylinders are not all firing properly
Fuel related is the most often problems, but could be spark or timing

Easy to test compression, spark, and timing.
then you are left with fuel to air ratio getting to cylinders to find

 

[Zac Penn]

Thanks for the reply. I will check those things on Monday and report back.

 

[Zac Penn]

Well I did the scary part today and all six cylinders had within 2 psi of each other so that is a relief!!!!
Do you happen to have the timing values handy that i can look at? I have a timing light but I don't know what the values should be at the different throttle openings.
As for testing the spark...
If my problem is only in the 1200-2000 rpm range how do I test that?

Thanks for the help,
Zac

 

[99yam40]

Sorry, but no I do not have the timing specs. Service manual should have what you need.

An inductive timing light can be used to see if current it being delivered to each of the spark plugs.
Some may have RPM readings also so you can compare to the other cylinders to see it there is a difference